I am a total beginner with Virtual Box and Turnkey. I know that what I post will necessarily evoke the reaction along the lines of 'nothing happens without you changing something' but to me it feels like that happened. My Turnkey VM was running just fine on Virtual Box yesterday but this morning I powered on my computer (MacBook Pro 13") and when I booted my Turnkey VM I was promted to configure my network details. When I was trying to do this automatically it would tell me that there was an error obtaining an IP address and that it didn't get a lease...

What happened? There is really NOTHING that I remember changing. I tried setting the IP manually but although this is 'technically successful' the VM does not respond when I type in its IP in my browser. Where should I start looking? As I said I have little experience and it would already help me if you could tell me the five most common fixes to this problem....

But it sounds like the virtual NIC is perhaps being blocked. The fact that you can't get a DHCP IP (and you could previously) and your VM is unreachable when you set a static IP suggets to me that there is no network traffic. Perhaps some sort of firewall is running? Some other (host) networking issue? Double check that networking is set to bridged.